This invention relates to a method for the recovery of palladium. More particularly, the present invention relates to a method for recovering palladium from spent catalysts.
In recent years, the chemical industry has unearthed a wide variety of applications for the noble metal catalysts. Unfortunately, these catalysts are costly and regeneration of spent catalysts is a necessity. Thus, there have arisen for this purpose numerous processes to enhance the effective use of these catalysts. However, regeneration is self-limiting in that a point is reached at which regeneration is no longer effective. Then, workers in the art focus their interest upon recovery of the noble metal, and it is to this end that the subject application is addressed.
Spent palladium catalysts typically comprise palladium in an amount ranging up to 2 percent, the remainder thereof comprising activated carbon, alumina and diverse impurities such as superfine sand, metal soaps and high molecular weight substances. Among the more popular procedures for recovery of palladium from spent catalysts are those which involve incineration of the catalyst in an oven to yield an ash having a palladium concentration ranging from 15 - 25 percent and a small percentage of carbon, frequently in the form of graphite. Unfortunately, the presence of carbon impairs the dissolution of the ash in acids and, consequently, must first be removed. This end is typically attained by annealing the ash at a temperature within the range of 600.degree. - 1000.degree. C. However, some of the carbon remains in the ash so treated, thereby preventing quantitative dissolution of palladium in aqua regia or hydrochloric acid admixed with hydrogen peroxide.
Nonetheless, after stirring the resultant solution, it is permitted to clarify and is then decanted. The sediment is next subjected to repeated dissolution and decantation and the extracts united. The residue is then filtered, washed, dried and treated with sulfuric acid. Thereafter, it is diluted with water, decanted and filtered. The filtrates so obtained are then reduced with zinc to yield raw palladium which is dissolved in an acid to form a concentrated palladium solution which is finally processed in a two-stage process to a palladium sponge. The sponge may then be washed and dissolved to yield a solution which is diluted to approximately 120 gram/liter palladium. In this manner, about 99 percent palladium is obtained from the ash.
As noted from a review of the conventional process described above, it is a time-consuming operation which places heavy demands upon labor and is objectionable from the standpoint of safety and hygiene. Additionally, the process does not permit the entire palladium content in the spent catalyst to be reclaimed.